The present invention relates, in general, to sensor devices and, more particularly, to a method for setting the sensing polarity of a sensor device, such as a pressure sensor.
Pressure sensors use a sensing die having a front side, which has most of the die's electronics formed thereon, and a back-side. Different sensing applications often require that either the front side or the back side of the sensing die be exposed to the media to be sensed. This requirement is dictated by factors, for example, such as the harshness of the media or the particular package design used to assemble the sensing die into a final, packaged sensor device.
The polarity of the sensing electronics of the pressure sensor are set depending on whether the front side or the back side of the sensing die will be used for sensing. In the past, this polarity was set by the metal mask used to form, for example, an aluminum interconnect level connecting the sensing element, such as a thin-membrane transducer, to amplifying electronics on the sensing die. However, because additional processing and assembly follows this metal mask process step, there is a significant time lag between the time of setting polarity and the completion of the sensor device.
The market for sensor devices is highly segmented, and the market demand, even for devices using the same particular type of sensing die, varies between front-side and back-side applications. Thus, because of the processing time lag above, it is necessary to hold excessive inventories of sensor devices, which are processed in advance with sensing polarities set for either front-side and back-side applications, in order to meet this variable demand in a timely manner.
Holding excessive inventory, however, leads to increased costs due to higher scrap rates for devices assembled in advance and not sold, increased manufacturing complexity because of the need to maintain two different mask sets (each set corresponds to a front-side or back-side application), and additional clerical work to track and monitor different lots of sensing dies pre-processed for either a front or back-side application. Further, the setting of the sensing polarity at the metal mask process stage results in the need for duplicate testing programs for ensuring operability of the fully-processed sensing die.
Thus, there is a need for an improved method for setting the sensing polarity of sensor devices that does not require the holding of excessive inventories of devices with sensing polarities pre-set at the metal mask step for either front or back-side applications. This need is driven by a desire to reduce manufacturing costs, to reduce customer order lead times, and to avoid the need for a manufacturing distinction between front and back-side applications until the trim-and-test stage, which follows assembly and packaging.